1. Field of the Invention
The present invention relates to an improvement in an automotive cooling system and, more particularly, to a hydraulic system for driving a cooling fan for a heat exchanger on an automobile.
2. Description of the Prior Art
A cooling system for vehicles has heretofore been known which includes a hydraulic motor for driving a cooling fan. For example, Japanese Unexamined Utility Model Publication No. 58-142,314 discloses a hydraulic system which includes an engine-driven hydraulic pump operative to circulate a hydraulic fluid through a hydraulic driving circuit, a hydraulic motor disposed in the circuit and a valve disposed in series or parallel relationship to the hydraulic motor and responsive to signals such as an air-conditioner signal and a water temperature signal to control the rate of the fluid supply to the hydraulic motor.
In the hydraulic system disclosed in the Japanese publication referred to above, however, if the automotive engine is stopped with the cooling fan and the hydraulic motor both rotated at high speeds, the rate of the hydraulic fluid supply to the hydraulic motor is suddenly decreased to zero. The rotations of the cooling fan and the hydraulic motor are continued due to inertia. Although the speeds of the rotations are gradually lowered, the rotating hydraulic motor now functions as a pump which sucks the fluid and then discharges the same. However, because the fluid supply to the hydraulic motor is now interrupted, a depression is created in the inlet side of the hydraulic motor, so that a cavitation is caused in the hydraulic motor and in the part of the hydraulic circuit connected to the motor to raise problems of the occurrence of noise and damage to the hydraulic motor.
In addition, when an automobile which is equipped with the cooling system is driven at a high speed and the ram causes the cooling fan to rotate freely at a speed higher than a predetermined value, a cavitation is also caused in the cooling system to not only produce noises but also generate a resistance to the free rotation of the cooling fan. The resistance reduces the usefulness of the ram.